While older chromate treatment baths consisted simply of aqueous solutions of chromic acid or dichromic acid, in recent years various improved methods have been proposed in which the chromate treatment bath lays down a film which is only slightly soluble in acid or alkaline treatment liquid compositions which may follow chromate film formation. Examples of this relatively recent art will be considered below.
The teaching of Japanese Patent Application Laid Open [Kokai or Unexamined] Number 50-158,535 [158,535/75] concerns a method for the formation of a slightly soluble chromate film on the surface of zinc coated steel sheet. A chromate bath is disclosed which is based on chromic anhydride (CrO.sub.3)+phosphoric acid (H.sub.3 PO.sub.4)+water soluble or water dispersible polymeric compound. At least 70% of the hexavalent chromium ion in this treatment bath is reduced by a reductant such as ethylene glycol or the like. However, since the chromate films formed according to the examples of this invention contain polymer, they suffer from a poor weldability although they are excellent with regard to lack of solubility, corrosion resistance, and adhesion to paint and corrosion resistance after painting (the last two characteristics being sometimes briefly denoted hereinafter as "coatability").
The chromate bath disclosed in Japanese Patent Publication Number 61-58522 [58,522/86] is a chromic acid (CrO.sub.3) +chromic acid reduction product+silica sol system. The major disadvantage with the method according to this invention is the tendency for the chromium, chiefly the hexavalent chromium, in the chromate film to elute during the alkaline rinse which is carried out after chromating but before the treated steel sheet carrying the chromate film is painted. This results in a decline in the film's corrosion resistance.
Japanese Patent Application Laid Open Numbers 58-22383 [22,383/83] and 62-83478 [83,478/87] disclose the use silane coupling agent in order to reduce the hexavalent chromium ion in the chromate treatment bath. Each of the films formed by the methods according to these inventions provides an excellent paint-film adherence. However, the chromate film produced by the method of the first invention has a poor alkali resistance. The alkali resistance is similarly unsatisfactory in the case of the method according to the second invention.